Wear boots. Balance on a rail or something of the nature. Figure out where your center of balance is so you have equal effort wiggling front to back. Mark with some gaffer tape, spraypaint or the girlsfriends lipstick depending on your flavor. Put boot in binding, look down board and see if it is centered. There will be an angle so both marks must be equally distant from the center of course.

Or, just step on the board next to the binding and figure out what feels good and puts equal pressure tow and heel side.